In recent years, a technique to manufacture a thin film transistor (Thin Film Transistor, hereinafter referred to as a “TFT”) over a substrate has made great progress and application development to, for example, an active matrix display device has been advanced. In particular, a TFT formed using a crystalline semiconductor film is superior in field-effect mobility (also referred to as mobility) to a TFT formed using a conventional amorphous semiconductor film; therefore, high-speed operation is possible. For this reason, it is tried that a pixel, which has been controlled by a driver circuit provided outside a substrate, is controlled by a driver circuit formed over the same substrate as the pixel.
A substrate used in a semiconductor device is expected to be a glass substrate rather than a quartz substrate in terms of cost. The glass substrate is inferior in heat resistance and easy to be deformed due to heat. Therefore, when the TFT using the crystalline semiconductor film is formed over the glass substrate, laser annealing is employed to crystallize a semiconductor film in order to prevent the glass substrate from being deformed due to the heat.
Compared with another annealing method using radiant heat or conductive heat, the laser annealing has characteristics that the processing time can be shortened drastically and a semiconductor substrate or a semiconductor film is heated selectively and locally so that thermal damage is hardly given to the substrate.
The laser annealing method described here indicates a technique to crystallize an amorphous layer or a damaged layer formed in a semiconductor substrate or a semiconductor film, a technique to crystallize an amorphous semiconductor film formed over a substrate, and a technique to heat (anneal) a crystalline semiconductor film which is not single crystal (the above-mentioned semiconductor films which are not single crystal are collectively referred to as a non-single crystal semiconductor film) by a laser beam. Further, a technique applied to flatten or modify a surface of the semiconductor substrate or the semiconductor film is also included.
In the laser annealing, a laser beam (also referred to as a laser beam) oscillated from an excimer laser is often employed. The excimer laser has advantages of high output and capability of repetitive irradiation at high repetition rate, and moreover, a laser beam oscillated from the excimer laser has high absorption coefficient to a silicon film which is often used as a semiconductor film.
At the irradiation of a laser beam, a method in which the laser beam is shaped into linear on an irradiation surface with an optical system and an irradiation position of the laser beam is moved in a short-side direction of the linear beam relative to the irradiation surface is high in productivity and superior industrially (refer to Patent Document 1).
[Patent Document 1] Japanese Published Patent Application Laid-Open No.: Hei8-195357